Lindsay Lohan has a half sister

Lindsay Lohan has been dealt a shock after a televised paternity test 'proved' long-running rumours her dad Michael has another daughter.

Kristi Horn has spent four years trying to convince Michael Lohan he is the father of her 17-year-old daughter Ashley, and he agreed to settle the matter by taking a DNA test on U.S. daytime TV's The Trisha Goddard Show.

In the big reveal, which will air in America on Thursday, Michael comes face-to-face with his alleged lovechild for the first time as Goddard reads out the positive result, prompting Kristi to break down in tears.

Michael, who had previously denied the allegation, stands up to hug the teenager but she tries to resist, saying, "This is the first time I've met you. I'm sorry, but I'm not sure..."

He replies, "It's OK. Everything in time, honey."

Born-again Christian Michael has four children with Dina Lohan, including Mean Girls star Lindsay, and is reportedly expecting another with girlfriend Kate Major.